Fire sensor may include a light transmitter and light receiver in a light-scattering arrangement having a light-scattering center located in the open outside the light-scattering smoke detector. The fire sensor may also be an optical measuring chamber that is arranged in a detector housing, is shielded from ambient light and is permeable to smoke to be detected. In addition, the fire sensor can comprise one or more temperature sensors. Such a temperature sensor may be, for example, a temperature-dependent resistor (thermistor), for instance what is known as an NTC or PTC, or a non-contact temperature sensor comprising a thermopile or microbolometer.
A fire detector typically also comprises a control unit, preferably a microcontroller. The control unit analyzes a sensor signal received from the fire sensor for at least one characteristic fire parameter, to evaluate said signal and to output a fire alarm on a fire being detected. A characteristic fire parameter may include, for a light-scattering smoke detector, exceeding a minimum scattered-light level which correlates to a smoke-particle concentration. Alternatively or additionally, an inadmissibly high rise in level of the scattered light may also be a characteristic fire parameter. In the case of a thermal detector, a characteristic fire parameter may include exceeding a minimum temperature in the (immediate) surroundings of the fire detector, for instance a temperature of at least 60° C., 65°, 70° C. or 75° C. Alternatively or additionally, a characteristic fire parameter may also be an inadmissibly high rise in temperature, for instance of at least 5° C. per minute or at least 10° C. per minute.
EP 2093734 A1 and EP 1039426 A2, for example, disclose open light-scattering smoke detectors. In addition, flame detectors are known from the prior art, for instance as disclosed by DE 10 2011 083 455 A1 or EP 2 251 846 A1. Such flame detectors are configured specifically for detecting open fire and for emitting an alarm in less than one second. They comprise usually two or more pyroelectric sensors as radiation sensors. Such sensors are tuned to detect characteristic flicker frequencies of open fire, i.e. flames and glowing embers, in the infrared region and, if applicable, in the visible and ultraviolet region. The flicker frequencies typically lie in a range of 2 Hz to 20 Hz.
EP 1039426 A2 discloses a smartphone having a fire-detector application comprising suitable program steps for analyzing video image data captured by an internal camera with regard to at least one piece of information characteristic of fire, and if said information is present, to output an alarm via an output unit. This smartphone is also configured to analyze the received video signal for the presence of flicker frequencies characteristic of open fire, and if there is a significant difference in two successive video images, to switch from a first, low image refresh rate to a second, high image refresh rate.
The infrared pyroelectric sensors are typically sensitive to infrared radiation in the wavelength range of 4.0 to 4.8 μm. This specific radiation is produced in the combustion of carbon and hydrocarbons. An example pyroelectric sensor is sensitive to characteristic emissions of metal fires in the UV region. For use in the open, flame detectors may also comprise a radiation sensor that is sensitive to infrared radiation in the wavelength range of 5.1 to 6.0 μm. This radiation is primarily parasitic radiation such as, for instance, infrared radiation from hot bodies or sunlight. A more reliable assessment, i.e. whether or not it is an open fire, is possible on the basis of all the sensor signals.